The structure of a water-cooled and water-moderated nuclear reactor of the boiling water type is well known. (See, e.g., U.S. Pat. Nos. 4,548,785 and 5,118,464 to Richardson et al.) As depicted in FIG. 1, a boiling water reactor 2 (BWR) includes a reactor pressure vessel 4 (RPV) containing a nuclear reactor core (not shown) submerged in a coolant-moderator such as light water. The core, which is surrounded by a circular cylindrical shroud 6, includes a plurality of replaceable fuel assemblies (not shown) arranged in spaced relation between a top guide 8 and a core plate 10. The fuel bundle assemblies are supported at the top by the top guide 8 and at the bottom by the core plate 10. The core top guide provides lateral support for the top of the fuel assemblies and maintains the correct fuel channel spacing to permit control rod insertion.
The annular space between the RPV 4 and the shroud 6 forms the downcomer annulus 5. Water recirculates inside the RPV, flowing vertically downward through the downcomer annulus 5, around the bottom edge of core shroud 6 and then vertically upward through the fuel core inside the shroud. After passing through the water-steam separators (not shown), the separated liquid water then mixes with feedwater in the mixing plenum. In the conventional BWR, feedwater is admitted into the RPV 10 via a feedwater nozzle 12 and a feedwater sparger 14, which is a ring-shaped pipe having suitable apertures for circumferentially distributing the feedwater inside the RPV. The separated water/feedwater mixture then returns to the core via the downcomer annulus. The steam is withdrawn from the RPV via steam outlet 22.
The BWR also includes a coolant recirculation system which provides the forced convection flow through the core necessary to attain the required power density. A portion of the water is sucked from the lower end of the downcomer annulus 16 via recirculation water outlet 24 and forced by a centrifugal recirculation pump (not shown) into jet pump assemblies 26 (only one of which is shown) via recirculation water inlets 28. The BWR has two recirculation pumps, each of which provides the driving flow for a plurality of jet pump assemblies circumferentially distributed around the shroud 6.
A core spray nozzle 16 supplies water to a core spray sparger 18 via core spray line 20 (see FIG. 1) in the event the emergency core cooling is required, e.g., in response to a loss-of-coolant accident. Installation of BWR 3/4/5 core spray lines inside the RPV requires initially that the core spray line 20 and T-box assembly 30 (see FIG. 2) be fitted to the thermal sleeve 32 in the core spray nozzle 16. The T-box 30a is then welded to the thermal sleeve 32 from the inside through access provided by the T-box front cover 30b. The resulting T-box to thermal sleeve weld 34 is creviced, as seen in FIG. 3. Finally, the front cover 30b is welded to the T-box 30a. The resulting front cover weld 36 is also creviced. The material of the T-box, front cover, thermal sleeve and associated welds is austenitic stainless steel having normal carbon content. Thus, some residual weld stresses can be expected. Therefore, the mechanisms are present for circumferential welds 34 and 36 to be susceptible to intergranular stress corrosion cracking (IGSCC).
As is evident from the foregoing, T-box assembly 30 is part of the emergency core cooling system and any defect (e.g., cracks) therein can jeopardize the proper operation of that system. Under certain conditions, the T-box assembly could undergo stress corrosion cracking in the heat-affected zones adjacent to the crevice welds 34 and 36. During core spray operation, complete failure of the thermal sleeve attachment weld 34 could lead to the opening-up of a gap between the T-box 30a and the thermal sleeve 32, thereby spilling some of the core spray flow out the gap. Since the gap is outside of the core shroud 6, the spilling water would be lost from both spray and reflood injection in the postulated case of a recirculation pipe rupture below the core elevation. In the case of the T-box front cover plate 30b exiting its position due to IGSCC, complete failure of that half of the core spray system could be expected.
Thus, the creviced welds 34 and 36 need to be examined periodically to determine their structural integrity and the need for repair. Ultrasonic inspection is a known technique for detecting cracks in nuclear reactor components. However, the core spray T-box welds are inherently difficult to access. Therefore, means for remotely and automatically inspecting the core spray T-box welds are needed.